This invention relates to thread trimming mechanisms, and more particularly, to an underbed thread trimming mechanism for sewing machines.
In the operation of an underbed thread trimming mechanism, two successive stages are required, each of which is critical in a different way. First, provision must be made for engaging or insuring that engagement will occur of the trimming mechanism with both the needle and looper threads of the sewing machine while these thrads extend beneath the work supporting bed of the machine; and second, a cutting action must be effected after the sewng threads have been sufficiently freed from the stitch forming instrumentalities as to make thread severance feasible. The first stage of underbed thread trimmer operation is usually the most critical insofar as timing is concerned since it requires motion of elements of the trimmer in close proximity to moving parts of the stitch forming instrumentalities. The second stage of underbed thread trimmer operation involves forces of a higher degree of magnitude incident to the drawing of thread under tension and the cooperative movement of the thread cutter elements.
To provide for one or both of these different stages of underbed thread trimmer operation, prior known mechanisms have utilized cams driven by the sewing machine. Such cam drives, however, are not only complex, costly, and not readily retrofittable on sewing machines, but they require a sewing machine drive motor capable of producing a higher torque. It is known to actuate an underbed thread trimmer by means of a double acting solenoid or two separate solenoids, both of which multiply the expense and complexity of the trimmer installation. Single acting solenoid actuated thread trimmers are also known in which the solenoid is energized to shift a thread engaging element into distended position in opposition to a spring which serves to return the element into thread cutting relation with a ledger blade. Relatively high magnitudes of force are necessary with this type of trimmer actuation with consequent increased expense.